VLBA observations of a rare multiple quasar imaging event caused by refraction in the interstellar medium
Aims. We have investigated highly atypical morphological parsec-scale changes in the flat spectrum extragalactic radio source 2023+335 which are coincident with an extreme scattering event (ESE) seen at radio wavelengths during the first half of 2009. Methods. We used (i) 15.4 GHz Very Long Baseline Array (VLBA) observations of the quasar 2023+335 obtained at 14 epochs between July 2008 and Nov. 2012 as part of the Monitoring Of Jets in Active galactic nuclei with VLBA Experiments (MOJAVE) program; (ii) earlier archival VLBA observations of the source performed at 1.4, 2, 8, 15, 22, and 86 GHz to analyze the properties of the proposed turbulent screen toward 2023 + 335; and (iii) data sets from the Owens Valley Radio Observatory (OVRO) and University of Michigan Radio Astronomy Observatory (UMRAO) single-dish monitoring programs performed at 15 and 14.5 GHz, respectively, to study integrated flux density changes. Results. We report on the first detection of the theoretically-predicted rare phenomenon of multiple parsec-scale imaging of an active galactic nucleus induced by refractive effects due to localized foreground electron density enhancements, e.g., in an AU-scale plasma lens(es) in the ionized component of the Galactic interstellar medium. We detected multiple imaging in the low galactic latitude (b = -2.°4) quasar 2023+335 from the 15.4 GHz MOJAVE observations when the source was undergoing an ESE. While the parsec-scale jet of the source normally extends along PA ~ −20°, in the 28 May 2009 and 23 July 2009 images a highly significant multi-component pattern of secondary images is stretched out nearly along the constant galactic latitude line with a local PA ≈ 40°, indicating that the direction of relative motion of the plasma lens is close to orbital. Weaker but still detectable imaging patterns at similar position angles are sporadically manifest at several other epochs. Modeling the ESE that occurred in early 2009 and lasted ~0.14 yr, we determined that the foreground screen has a double-lens structure, with proper motion (~6.8 mas yr^-1), and angular size (~0.27 mas). We also found that the angular separation between the two brightest sub-images roughly follows a wavelength-squared dependence expected from plasma scattering. Furthermore, by analyzing archival non-simultaneous VLBA observations covering a wide frequency range from 1.4 to 86 GHz, we found that the scattered angular size of the VLBI core follows a ν^(-1.89) dependence, implying the presence of a turbulent, refractive dominated scattering screen that has a confined structure or is truncated transverse to the line of sight toward 2023+335.
Additional Information© 2013 ESO. Article published by EDP Sciences. Received 15 March 2013; Accepted 22 May 2013. The authors acknowledge K.I. Kellermann, E. Clausen- Brown, and the other members of the MOJAVE team. We are also grateful to J. Cordes, B. Rickett and T. J. W. Lazio for productive discussions. We thank the anonymous referee for useful comments which helped to improve the manuscript. This research has made use of data from the MOJAVE database that is maintained by the MOJAVE team (Lister et al. 2009a). The MOJAVE project is supported under NASA Fermi grant 11-Fermi11-0019. Part of this work was supported by the COST Action MP0905 "Black Holes in a Violent Universe". A.B.P. was partially supported by DAAD and the "Nonstationary processes in the Universe" Program of the Presidium of the Russian Academy of Sciences. Y.Y.K. was supported by the Russian Foundation for Basic Research (projects 11-02-00368, 12-02-33101), the basic research program "Active processes in galactic and extragalactic objects" of the Physical Sciences Division of the Russian Academy of Sciences, and the Dynasty Foundation. T.H. was supported by the Jenny and Antti Wihuri foundation. E.R. was partially supported by the Spanish MINECO projects AYA2009-13036-C02-02 and AYA2012-38491-C02-01 and by the Generalitat Valenciana project PROMETEO/2009/104. The UMRAO monitoring program is supported in part by NASA Fermi GI grants NNX09AU16G, NNX10AP16G and NNX11AO13G and NSF grant AST-0607523. The OVRO 40-m monitoring program is supported in part by NASA grants NNX08AW31G and NNX11A043G, and NSF grants AST-0808050 and AST-1109911. The VLBA is a facility of the National Science Foundation operated by the National Radio Astronomy Observatory under cooperative agreement with Associated Universities, Inc. This research has made use of BM046, RDV26,40, BM290A-E, BR149, BM167, BM231, GM064, BR145D experiment data from the public VLBA archive of correlated data. This work made use of the Swinburne University of Technology software correlator (Deller et al. 2011), developed as part of the Australian Major National Research Facilities Programme and operated under licence.
Published - aa21484-13.pdf
Submitted - 1305.6005v1.pdf